JPH0645695A - Patterned mirror vertical-type cavity surface emission laser - Google Patents

Abstract

PURPOSE: To provide a top emission type VCSEL which is newly improved by a simple manufacture, and enable large output power and high efficeincy. CONSTITUTION: This VCSEL contains a central active layer 16, having an upper mirror lamination part 18 and a lower mirror laminating part 14. A circular trench 20 is formed in the mirror laminating part 18 and defines a laser irradiation region 25. The trench 20 decreases reflectivity and prevents laser irradiation outside the operating region 25. Oxygen injection material 27 in the trench 20 restricts current distribution and maxmizes power output and efficiency. By means of the trench 20, self-alignment is enabled through the grater part of manufacturing processes.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION This invention relates to vertical cavity surface emitting lasers (VCSELs). More specifically, it relates to VCSELs having higher power and greater efficiency.

[0002]

BACKGROUND OF THE INVENTION Conventional edge emitting semiconductor lasers play an important role in the development of optical communications due to their high operating efficiency, small size and modulation capability. However, as performance needs grow, dimensions shrink, and manufacturing needs grow, these devices are reaching their limits.

Recently, a vertical cavity surface emitting laser (VC)
There is a growing interest in a new type of laser device called SEL). The advantages of VCSEL devices are that they are smaller, have the potential to have higher performance, and may be more productive. These advantages are due, in part, to the development of epitaxial deposition techniques such as metal organic vapor phase epitaxy (MOVPE) and molecular beam epitaxy (MBE).

However, even with these advances in deposition technology, it is difficult to control the mode of operation of the laser during manufacture and to control the current distribution within the laser. Generally, a VCSEL is created by depositing multiple layers on a substrate and then etching the layers down to the depth of the substrate that will form the VCSEL. For example, US Pat. No. 5,034,092 “Plasma Etching of Semicon,” which was assigned to the same assignee as the subject matter of July 23, 1991 and is also incorporated herein by reference.
See ductor Substrates ”.

The etching of mesas to make VCSELs has two drawbacks. The etching process damages the surface crystals, which increases the threshold current,
It is less reliable. The mesas form a waveguide with a large discontinuity in the index of refraction, which makes it difficult to control the optical modes without making very small devices, which increases series resistance and Reduce output power. In general, this results in less efficient and less stable devices.

[0006]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a new and improved VCSEL which is easier to manufacture.

Another object of the present invention is to provide a new and improved V which produces higher output power and has higher efficiency.
It is to provide CSEL.

First and second layers deposited on both sides of the active layer
The above and other problems are solved and the objectives achieved by a method of making a vertical cavity surface emitting laser including a mirror stack of. This method includes the steps of defining an operation region in the second mirror stack portion, etching the second mirror stack portion to form a trench surrounding the operation region, and sufficiently extending the depth of the trench. , A step of reducing the reflectivity below that required to irradiate the laser volume between the trench and the active layer.

Furthermore, surface emitting lasers solve the above and other problems and achieve the objectives. The surface emitting laser includes an active layer having opposed major surfaces, a first mirror stack on one major surface of the active layer, and a second mirror stack on another major surface of the active layer. , The second mirror stack has an operating region defined by a trench extending from the main surface of the second mirror stack towards the active layer, the trench extending deep enough around the operating region. Thus lowering the reflectivity below the amount required to accommodate laser irradiation for the laser volume between the trench and the active layer.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring particularly to FIG. 1, a vertical cavity surface emitting laser 10 is shown.
Are shown in cross section. Laser 10 is formed on a substrate 12, which in this example is made of n-type doped gallium arsenide. Gallium arsenide is used as the substrate 12 to promote epitaxial growth of multiple layers of gallium arsenide and aluminum arsenide. It should be understood that other semiconductor substrates can be used as well.

Epitaxial deposition of different compositions in alternating layers is accomplished by known techniques such as MBE, MOCVD. These techniques enable the epitaxial deposition of relatively thin or thick layers of various materials such as gallium arsenide, aluminum gallium arsenide / gallium, aluminum arsenide and indium phosphide. VC
Epitaxial deposition is widely used in the fabrication of SEL devices to create multiple layers of different materials from which the device is built.

By depositing alternating layers of doped gallium arsenide and aluminum arsenide, a first stack 14 of reflectors or mirrors for laser 10 is formed. The thickness of the alternating layers of gallium arsenide and aluminum arsenide is approximately 1/4 of the wavelength at which the device is designed to operate.
Is set to. The gallium arsenide and aluminum arsenide layers have nominal thicknesses of 665 Å and 80, respectively.
It is about 0 angstrom.

The conventional active layer 16 is formed as follows. A cover region is epitaxially deposited on the first mirror stack 14 to form a first major surface of the active layer 16. The covered area usually has two parts, but these are not shown to avoid overcrowding FIG. The first is a silicon-doped aluminum gallium arsenide layer, having a thickness of about 800 Å, deposited on the first mirror stack 14. The second is an undoped aluminum gallium arsenide layer, about 500 angstroms thick, deposited over the silicon-doped aluminum gallium arsenide layer. An active area is epitaxially deposited on the coated area 13, which is normally center strained.
) Made of two barrier regions attached to opposite sides of the quantum well region. The two barrier regions are made of undoped gallium arsenide each about 100 angstroms thick. Multiple quantum wells and barriers can also be used to increase the available gain. The strained quantum well region is typically made of undoped indium gallium arsenide about 80 angstroms thick. The second cover region is epitaxially deposited to form the second major surface of active layer 16. The second covered area usually consists of two parts. First, about 500 angstroms of undoped gallium aluminum arsenide is deposited on active region 17. Next, about 800 Å of beryllium-doped aluminum gallium arsenide is deposited on the undoped aluminum gallium arsenide.

A second stack 18 of reflectors or mirrors.
Are epitaxially deposited on the second coating region of the active layer 16. The second mirror stack 18 is composed of alternating layers of beryllium-doped gallium arsenide and aluminum arsenide. The thickness of the last alternating layer is about 1/2 of the wavelength, not 1/4 of the wavelength used for the other alternating layers.
Is. Generally, the threshold current of a VCSEL is reduced by increasing the number of alternating layers in the mirror stacks 14, 18.

Once the first mirror stack 14, the active layer 16 and the second mirror stack 18 are complete, the structure must be patterned to form one or more individual VCSELs. In this particular embodiment, patterning is performed as follows. Second mirror stacking section 18
The top surface of the is coated with a layer of photoresist material by any known method. The photoresist layer is exposed and the material is removed to define the location and dimensions of trench 20. A trench 20 is then formed by etching the mirror stack 18 by ion milling or the etching process disclosed in the '092 patent shown above. In general, the trench 20 completely surrounds and defines the active area 25. It has a generally circular cross section in this particular embodiment.

In this particular embodiment, trench 2
0 indicates the first mirror laminated portion 14, the active layer 16 and the second mirror laminated portion 18 from the upper surface of the mirror laminated portion 18 to the inside thereof.
It extends to a depth of approximately one-half the overall size. This depth is expedient for reasons that will become apparent below, but the trench 20 is defined by the bottom of the trench 20 and the active layer 16.
It suffices to make the mirror laminated portion 18 deep enough to reduce the reflectivity in the volume between and to prevent laser irradiation in the volume. In at least some applications, laser irradiation will not occur once the reflectivity drops below about 98%. Note that the optical mode of operation is generally determined by the depth to which trench 20 is etched.

Operating region 25 surrounded by trench 20
Is basically a mesa with a generally circular cross section.
The operating wavelength (frequency) of the laser 10 is already the active layer 16
Structure (materials, etc.) and dimensions, and the mirror laminated portion 14, 1
8 dimensions. Further, as is well known in the art, it is desirable for a VCSEL to operate in the lowest order mode. As such, the size of the mode, ie, the maximum volume available to maintain the lowest order mode of operation, is readily determined. FIG. 2 is a graph showing a comparison of the laser irradiation modes of the laser 10 and their surface areas. The distance D is generally the maximum area that corresponds to operation in the lowest order mode without including higher order modes.

In the manufacturing method of the present case, the mode size for the lowest order mode is predetermined, and the diameter of the operation area 25 is set to be the same. Masking the structure for etching the trench (or trenches) 20 is not important, since the laser irradiation only occurs in the volume below the active region 25. Generally, the depth of the trench 20 is such that it does not contact the active layer 16, thereby improving reliability. Also, the width of the trench 20 is not critical and may be any convenient width depending on the application and the manufacturing steps below.

The operating region 25 of the laser 10 is set substantially equal to the mode dimension of operation, ie, the lowest order mode, so that the laser 10 output or emission point is maximized. When the emission point of the laser 10 is maximized,
Its output power is also maximum. Trenches 20 are formed to the desired depth and an oxygen implant layer 27 is formed at the bottom thereof,
Annealed to form a high resistance region for current flow. Layer 27 limits the current in laser 10 to generally the volume in line 30. Also, line 30 is typically a trench 20.
Defines the volume of the laser 10 at which laser irradiation occurs due to the reflectance reduction produced by. By controlling the current distribution only in the desired laser irradiation region, the wasted current is minimized and the efficiency of the laser 10 is maximized. Furthermore, by increasing the operation area while increasing the threshold current, the series resistance is reduced and the output power is further increased. The oxygen implant layer 27 controls the spread of current regardless of the depth of the trench 20.

To complete the structure of the laser 10, a layer 32 of SiNx, which is any known silicon-nitrogen compound such as silicon nitride, is provided over the entire upper surface of the second mirror stack 18, the trench 20, and the operating region 25. To adhere to. Layer 3
2 may be any convenient masking / insulating layer to protect the surface of laser 10. A substantially planar photoresist layer is then applied over layer 32. The photoresist layer is thicker in the trench 20 and can be removed from the active region 25 using only coarse alignment. The portion of SiNx layer 32 over active area 25 is then removed and laser 10 is patterned for p metallization lift-off and bond pads as indicated by metal layer 34. A second metal layer 36 is deposited on the underside of laser 10, which is the underside of substrate 12 in this example, to form a second electrical contact. Usually metal layers 34, 36
Is made of titanium, platinum and gold, and the metal layer 36 is made of nickel, germanium and gold. Metal layers 34, 36
Are created so that the geometric pattern is formed using a conventional lift-off process. It should also be appreciated that other masking structures and methods can be used to create geometric patterns such as photoresist, dielectrics, and the like.

Thus, the self-alignment of the mirror etch and the oxygen implant, and the dielectric Vid on the mesas that are virtually self-aligned.
New and improved VCSE easy to manufacture due to etch
L is disclosed. In addition, the optical mode control provided by the trench and the current distribution control provided by the oxygen implant maximize the power output and efficiency of the laser.
Finally, although the fabrication of a single laser is discussed herein, it should be understood that individual lasers, arrays of lasers, semiconductor wafers of lasers, etc. can be readily manufactured by the disclosed method.

[Brief description of drawings]

FIG. 1 is a cross-sectional view of a vertical cavity surface emitting laser incorporating the present invention.

FIG. 2 is a graph showing a laser irradiation mode of the laser of FIG. 1 in comparison with its surface region.

[Explanation of symbols]

 10 Laser 12 Substrate 14,18 Mirror Laminated Part 20 Trench 25 Operating Region 27 Oxygen Implant Layer 32 Silicon-Nitrogen Compound (SiNx) Layer

Claims (3)

[Claims] 1. A method of making a vertical cavity surface emitting laser (10): providing a substrate (12) having first and second opposing major surfaces; said first of said substrates (10). Depositing a first mirror stack (14) on the main surface; an active layer (16) on the first mirror stack (14)
Depositing a second mirror stack (18) on the active layer (16); defining an operating region (25) in the second mirror stack (18); and The two-mirror stack (18) is etched to form a trench (20) surrounding the active region (25) and a laser (10) between the trench region (20) and the active layer (16). Extending the depth of the trench (20) in order to reduce the reflectivity sufficiently below the amount required for laser irradiation in the volume. 2. A surface emitting laser (10) comprising: an active area (16) having opposite major surfaces; a first mirror stack (14) on one major surface; and on the other major surface. A second mirror stacking portion (18) is provided with a mesa (25) having a circular cross section at least in a part thereof, and the laser (10) is a lowest order mode of operation having a predetermined mode size. A surface-emitting laser (10), characterized in that it comprises a second mirror stack (18) having an overall circular cross-section with a diameter substantially equal to a predetermined mode dimension. 3. A surface emitting laser (10) comprising: an active layer (16) having opposite major surfaces; a first mirror stack (14) on one major surface of the active layer; and the active layer. A second mirror stacking portion (18) on the other main surface of (16), wherein the second mirror stacking portion is the second mirror stacking portion.
From the main surface of the mirror stacking part (18) to the active layer (16)
Has an operating region (25) defined by a trench (20) extending toward, the trench (20) surrounding the operating region (25) and extending to a sufficient depth to provide the trench (20). 20) and the second mirror stack (1) which reduces the reflectivity below the amount required for laser irradiation within the volume of the laser (10) between the active layer (16).
8); a surface emitting laser (10).